1. Field of the Invention:
The present invention relates to a moisture content measuring system for analyzing, on an on-line basis, moisture content of coal in a coal burning boiler, of coke in a coal or iron manufacturing plant or of lime, coal or the like in a cement manufacturing plant.
2. Description of the Prior Art:
In most of the coal burning thermal power stations, many kinds of coal are used as mixed together. In order to efficiently burn coal and to smoothly perform a desmoking treatment operation, delicate operation control adapted to the nature of the coal is necessary. Normally, coal is stored as classified depending upon its brand, and the thermal power station is operated by determining a coal mixing proportion on the basis of results of analysis of the respective brands. However, since coal is solid, a broad distribution of quality exists even among the same brand of coal, and for an appropriate use of mixed coal and for combustion control, on-line analysis, from which results can be obtained with a high degree of precision and moreover quickly, is desired. Among other things, with regard to moisture in coal, a moisture content value would vary largely even in the same lot, as influenced by liquid sprinkled for preventing the scattering of coal dust during coal storage and by rainfall, and so, it is considered to be especially important to achieve on-line analysis.
A moisture content measuring process for coal is prescribed in JIS M8811 (Japanese Industrial Standard), 1976, and according to this process, a predetermined amount of sample coal representing a lot of coal is picked up. After the sample has been preliminarily dried at a temperature of 35.degree. C. or lower, it is pulverized and subjected to sample reduction. Then, a pulverized sample having a grain size of 3 mm or less or a grain size of 9.5 mm or less is held within a dryer until it assumes a constant weight at 107.degree. C., and a total moisture content is calculated from the total amount of reduced weight due to drying.
Regarding moisture content measuring apparatuses, other than an apparatus relying upon the above-mentioned JIS process, an infrared moisture meter making us of infrared rays, an electrostatic capacity type moisture meter making use of the correlation between a moisture proportion and a dielectric constant, a microwave moisture meter making use of the absorption of microwaves by moisture, a neutron moisture meter making use of the moderation and scattering of neutrons by hydrogen atoms, and the like are known. And other than with coal, such apparatuses are widely used as apparatuses for performing on-line analysis, such as in the measurement of a degree of dryness of cereals, papers, woods, etc.
In addition, a microwave heat-drying moisture meter, in which an article is dried by irradiating microwaves and a moisture content is calculated from a reduced weight, is sold in the market.
In the moisture measurement according to the abovementioned JIS process in the prior art, apart from the precision of analysis, normally a half day is necessary for measuring a total moisture content, and so, it takes far longer than 0.5-1 hours which is considered to be desirable for operation and control of a boiler. On the other hand, although the on-line analyzing processes in the prior art had an advantage that results of measurement could be obtained almost on a real-time basis, every one of the processes had a large measurement error and were not respectively practical in that the infrared moisture meter could measure only a moisture content in a surface portion, in that in the electrostatic capacity type moisture meter and the microwave moisture meter the influence of a bulk density upon the measured value was large, and in that in the neutron moisture meter there existed absorption by hydrogen atoms in coal besides the absorption by the moisture. In addition, while the microwave heat-drying moisture meter sold in the market was composed of a microwave source and a ventilation fan in combination, a water dammy circulating section in which water is circulated in a loop through the casing to absorb and exhaust excess energy from the microwave source, an automatic weighing section, a turntable, and a controller-calculator section for the system and it could dry a sample in 1-3 minutes and automatically perform a calculation to obtain a moisture content value, it involved the following problems.
(1) Since the object of measurement is an article having a relatively uniform distribution of moisture including foods such as milk, butter, cheese or the like and powdered or granular medicines, the moisture meter can be applied only to a sample amount of as little as 1-10 g; it cannot measure a large amount of sample (for instance, a sample of 1 kg or more) to which the present invention is directed.
(2) Heating a sample depends solely upon heating by microwave absorption. In the case of a sample of coal having a range of grain sizes as broad as several tens of micrometers to several tens of millimeters, and moreover, having different shapes and sizes, uniform heating is difficult and the sample is locally overheated midway during the moisture measurement. Hence, decomposition of coal and the like occurs, and therefore, an accurate moisture content value cannot be obtained.
(3) Accordingly, the above-mentioned moisture meter employs a system in which a drying period is controlled by a preset timer, and so, it is necessary that a drying time should be preliminarily known for the respective samples.